This study attempts to pharmacologically characterize axonal endings in trigeminal nucleus caudalis (CAUD). The methods employed include light and electron microscopic autoradiographic localization of labeled axonal endings in CAUD following application of tritiated (3H) putative neurotransmitters into CAUD. The transmitters to be investigated include serotonin, norepinephrine, GABA, dopamine, and glutamate. Our approach relies on the ability of neurons which normally utilize a specific neurotransmitter to take up this (3H) transmitter at their axonal endings. The purpose of these studies is to locate and characterize pharmacologically distinct axonal endings in CAUD which might be involved in trigeminal pain pathways.